1. Field of the Invention
The invention relates generally to safety and protection circuits, and in particular to ground fault protection circuits.
2. Description of the Related Art
A Ground Fault Circuit Interrupter (GFCI) is a device that is operable to de-energize a circuit in response to the detection of a ground fault condition at an AC load. The GFCI device has a pair of load side phase and neutral terminals, and also has a pair of line side phase and neutral terminals. The GFCI device is operable to enter a de-energized state to isolate an AC load connected to the load side terminals from the line side terminals upon detecting a ground fault condition. GFCI devices are usually designed according to accepted standards, such as UL 943, entitled “UL Standard for Ground-Fault Circuit-Interrupters”; UL 1053, entitled “UL Standard for Safety for Ground-Fault Sensing and Relaying Equipment”; and UL 1640, entitled “UL Standard for Safety for Portable Power-Distribution Equipment,” for example. These standards define various operating specifications and ratings of GFCI devices.
A GFCI device may be used in a single phase circuit, such as a single phase 120V AC circuit, or in a polyphase circuit, such as 120/240V AC circuit. The GFCI device typically detects the ground fault condition by sensing a current imbalance between the phase and neutral terminals caused by a ground fault current. When the current imbalance exceeds a threshold, the GFCI device enters the de-energized state that isolates the AC load from the line side phase and neutral terminals. The current imbalance threshold is typically defined by the class rating of the GFCI device. A Class A GFCI device, for example, trips when the ground fault current exceeds 5 mA, and a Class B GFCI device trips when the ground fault current exceeds 20 mA.
Known GFCI devices may malfunction when the line side power signal is not within a defined tolerance. For example, in the event of a damaged neutral line, such as in a 120/240 polyphase circuit, a Class A GFCI device may fail to provide rated protection. A load imbalance on the load side may cause the line to neutral voltage to likewise become imbalanced, resulting in an overvoltage or undervoltage condition that may cause the GFCI device to malfunction. Accordingly, the GFCI device may fail to provide its rated protection. Additionally, known GFCI devices may not provide protection from an overvoltage or undervoltage condition on the line side, as the GFCI device is typically designed to trip based on a ground fault condition, and is not designed to trip based on a overvoltage or undervoltage condition.